Phosphorus Fractions and Its Summer's Release Flux from Sediment in the China's Three Gorges Reservoir

The amounts and forms of phosphorus (P) in sediments, orthophosphate (Pi) concentrations as well as Pi release fluxes across the sediment-water interface were investigated from five sampling sites in the Three-Gorges Reservoir (TGR). The Total P (TP) contents of surface sediments (0-2 cm) ranged from 415.5 to 1047.9 mg/kg, and the predominant fraction was HCl-P (Ca-bound P). The main rank order of P-fractions was HCl-P > Res-P (Residual-P) > NaOH-P (Al-adsorbed P) > BD-P (Redox sensitive P) > NH4Cl-P (Labile P), and ranged from 164.5 to 687.2 mg/kg, 96.0 to 140.8 mg/kg, 75.8 to 111.5 mg/kg, 34.0 to 104.8 mg/kg, and 5.9 to 15.9 mg/kg, respectively. The pH values ranged from 6.4 to 8.2 across sediment-water interface and sediment pH values were lower than those of the corresponding overlying water. The average Pi concentrations of pore water were higher than those of the open water and overlying water. Based on calculations using the Fick's first law, sediment in Xiangxi River (XX03) functioned as a P sink, with Pi flux of -0.003 mg/m^2/d, while sediments from Modaoxi03 (MDX03), Daning02 (DN02), Caotanghe (CTH) and Changjiang Badong (CJBD), had Pi fluxes of 0.013 mg/m^2/d, 0.009 mg/m^2/d, 0.004 mg/m^2/d and 0.007 mg/m^2/d, respectively, and served as an internal P source in summer.